1. Field of Invention
This invention relates to new photosensitive polyimide compositions and simplified lithographic processes achieved by their use. More particularly, this invention is directed to the use of hydroxypolyimides and hydroxypolyimide precursors as high temperature stable imageable layers for use as photoresists and in the formation of imaged dielectrics.
2. Background Art
The invention relates to lithographic processes and materials for process simplification, reduced production cost and improved performance of multilayer metal structures in microelectronic fabrication. It is generally known in semiconductcr technology that there are several advantages in the use of polyimides as insulating or passivating material in electronic components. According to conventional techniques, a polyimide precursor is applied from a solution in a suitable solvent and the film formed is subjected to a bake/cure cycle to form the polyimide. Patterning of such film is carried out according to standard procedures involving dry etching or reactive ion etching (RIE) when using a multilayer resist process. Recently, there has been much interest in the development and utility of directly patternable or photosensitive polyimides which can be imaged according to the standard lithographic techniques. These materials are polyimide precursors which are imidized after they are patterned. This approach has the advantage of providing simplified process for the polyimide dielectric patterning and thus has the potential for reduced cost due to fewer number of operations. In addition to use as imageable dielectric, photosensitive polyimides are also attractive for application as high Tg and high temperature stable resist materials. The conventional resist compositions derived from novolak resins (phenol-formaldehyde resins) and diazonaphthoquinone based photoactive component are suitable only for low temperature metallization (80.degree.-90.degree. C.) due to the low Tg of novolak based systems. When the patterned resist layer is UV hardened to provide cross-linking, the upper limit for subsequent metallization process can be extended to about 150.degree. C. This increases processing difficulties since the hardened resist layer has to be stripped by dry etching, hot KOH or strong acid treatment which results in ionic contamination and can lead to defects and dislocations detrimental to device performance. Thus the novolak based photoresist systems containing diazonaphthoquinone sensitizers have limited high temperature application due to low Tg, low thermal stability, UV hardening required prior to high temperature processing.
In keeping with the need for thermally stable, high Tg resists which may also be used for permanent dielectric applications, several photosensitive imidizable formulations have become commercially available. Although the materials from different sources have different levels of radiation sensitivity and properties of fully imidized patterned films, most are based on Siemens technology, comprising acrylate functionalized polyamic ester or ammonium salt forming component and suitable photoinitiators. The olefinic pendant group crosslinks on exposure to UV radiation which results in increased molecular weight in the exposed region and thus pattern can be developed by selective removal of the unexposed region during subsequent solvent development. The resist is then converted to thermally stable polyimide pattern by further bake/cure cycle.
It has been generally observed that such photosensitive polyimides suffer from several limitations and are not suitable for high density integrated circuits. For example, there is 40-50% shrinkage in film thickness between image development and final cure to polyimide which results in distortion of pattern profile where the sidewalls collapse inwards as substrate adhesion prevents isotropic contraction. Other limitations are the resolution, shallow profiles, poor shelf life, and problem of contrast. These limitations are typical of this class of materials in general.
The development of photosensitive imidizable materials is typified by compounds and processes that have become known as "Siemens technology". These materials are typically compositions comprising polyamic acid esters with methacryloylethanol (hydroxyethylmethacrylate HOCH.sub.2 CH.sub.2 OOCC(CH.sub.2)CH.sub.3) or .beta.-oxyethylmethacrylate, and maleimide and a photoinitiator that react under the influence of radiation of the appropriate frequency and intensity to cause crosslinking through the hydroxyethylmethacrylate groups. The crosslinked polyamic ester intermediate remains insoluble during organic solvent development. The patterned crosslinked polyamic ester is then heat cured to about 400.degree. C. to cause dissociation of the aliphatic crosslinking groups and formation of the final thermally stable polyimide patterns.
As an alternative to the conventional photosensitive imidizable materials based on Siemens technology involves amine salt formulations instead of esters. Typical mixtures comprise a polyamic acid and a crosslinkable monomer amine such as dimethylaminoethylmethacrylate [(CH.sub.3).sub.2 NCH.sub.2 CH.sub.2 OOCC(CH.sub.2)CH.sub.3)] and a photoinitiator to function as photosensitive compositions which are processed similarly to the polyamic esters above.
In addition to the photosensitive polyimides based on Siemens technology, there have been reports describing preparation of photosensitive compositions by incorporation of photoactive compounds in standard polyamic acids. These formulations can be patterned according to conventional lithographic techniques using aqueous base developers and form positive tone patterns. However, this approach has not found practical application due to the problem that a very high percentage loading of photoactive compound is needed for creating any differential in the dissolution behavior of the exposed versus the unexposed area to allow pattern generation. This results in degradation of mechanical and electrical properties of the imidized patterned films. Also, resolution and image quality are less than satisfactory especially in the case of thicker films.
Various references broadly disclose photosensitive imidizable materials, but none describes the compositions or performance benefits of the material systems of our invention or the method of its application in the fabrication of microelectronics.
U.S. Pat. No. 4,661,435 discloses the preparation of photosensitive polyamic acid derivatives by a method which comprises the treatment of a polyamic acid with an isourea derivative that transfers the O-substituent of the isourea as a half-ester product with the polyamic acid. These materials crosslink in ultraviolet light without the necessity of a catalyst or photoinitiator. Films of these materials may be patterned and thereafter thermally treated to cause imidization and are thus useful to incorporate the resultant polyimides in semiconductor devices and the like.
U.S. Pat. No. 4,551,522 discloses the preparation of photopolymerizable polyamic acid derivatives comprising the partial derivatization of an aromatic dianhydride with a reactive monomer containing a photosensitive moiety selected from the group of ethylenically unsaturated alcohols, thiols and amines, condensation of the partially derivatized aromatic dianhydride with an aromatic diamine to form a polyamic acid, isoimidization of the polyamic acid with N,N'-dicyclohexylcarbodiimide or trifluoroacetic acid anhydride, mixing a molar excess of the photosensitive monomer with the polyisoimide in an aprotic solvent to convert the polyisoimide to the corresponding polyamic acid derivative.
U.S. Pat. No. 4,565,767 is directed to light-sensitive polymer compositions comprising a polyamic acid, a bisazide selected from 2,6-di(4'-azidobenzal)- and 2,6-di(4'azidocinnamylene)-4-substituted cyclohexanones, and a tertiary amine having an unsaturated side chain. After the relief patterns are formed by development of the exposed polyamic acid compositions, the patterned layer may be thermally converted to the corresponding polyimide.
U.S. Pat. No. 4,093,461 discloses positive-acting, thermally-stable photoresist compositions comprising a polyamic acid and an o-quinone diazide or a naphthoquinone diazide photoinitiator. These compositions may be imidized after patterning and development.
U.S. Pat. No. 4,395,482 discloses the preparation of photosensitive polybenzoxazoles precursors by the condensation of aromatic and/or heterocyclic dihydroxydiamino compounds with dicarboxylic acid chlorides and esters. These materials may be sensitized with light-sensitive diazoquinones. After exposure and development to form patterns, the patterned polybenzoxazole precursor material is converted to highly heat resistant polybenzoxazole by thermal treatment.
U.S. Pat. No. 4,803,147 discloses imageable polyimide compositions comprising a solvent soluble polyimide which is a condensation product of an aromatic dianhydride and an aromatic primary diamine wherein at least one of the dianhydride and diamine has a hexafluoroisopropylidene or 1-phenyl-2,2,2-trifluoroethane bridge, a photoinitiator (a diazine or triazine) and a photopolymerizable compound containing at least two terminal ethylenically unsaturated groups.
IBM Technical Disclosure Bulletin, Vol. 23, No. 10, pp. 4782-83 (March 1981) describes photoactive polyimide compositions made by the addition of about 6-15% by weight of a photoactive compound to a polyimide and their subsequent exposure, development and patterning. These materials are base developable to form positive images and the photoactive compound is typically a diazoquinone or the like.
D. N. Khanna and W. H. Mueller, "Photopolymers: Principles, Processes and Materials," Regional technical Conference Proceedings, p. 429-43, Oct. 30-Nov. 2, 1988, Ellenville, N.Y., discloses positive working photoresists based on hydroxy polyimides and hydroxy polyamides which contain hexafluoropropylidene (6F) linking or bridging groups. These materials are prepared using solution condensation techniques and are sensitized with photoinitiators such as diazonaphthoquinones. Hydroxypolyamides are synthesized by a low temperature condensation process and are then converted to the polybenzoxazoles of U.S. Pat. No. 4,395,482. The disclosed hydroxypolyimides are synthesized by the high temperature solution condensation process described in European Patent Publication 0 163 518. Use of this method causes chain degradation and leads to significant decreases in viscosity and molecular weight resulting in inferior polymer properties. In order to alleviate some of the drawbacks, it has been found to be desirable to formulate copolymers having chain units free from the hydroxydiamine end groups.
European Patent Publication 141,781 discloses a photosensitive polyimide composition comprising a polymethyl substituted soluble polyimide and a bisazide. Upon exposure, the exposed area undergoes crosslinking and becomes less soluble which allows image development by conventional solvent techniques. The presence of methyl groups is undesireable as it leads to poor thermal and oxidative stability.